The present invention related to electro-optical transducer modules and more specifically to an electro-optical transducer module having an inverted electro-optic guided-wave transducer.
Electro-optical transducer modules are a broad class of devices for generating or modulating optical signals in response to electrical signals. Electro-optical transducer modules generally have optical waveguide inputs and/or outputs for coupling optical signals into and/or out of the module. Within the module, the optical waveguides are accurately positioned relative to the electro-optical transducer for receiving the optical signals into or transmitting the optical signal out of the transducer. Generally the positioning accuracy between the waveguides and the transducer is in the tenth of microns. This accuracy requirement places severe constraints on the packaging design for these modules.
U.S. Pat. Nos. 4,772,586 and 4,997,253 describe one class of electro-optical transducer modules having lazing electro-optical transducers for generating optical outputs into optical fiber waveguides. In the '586 patent, the transducer module has a base member with a flat surface and a platform upstanding from the flat surface where the laser is positioned. The optical fiber is soldered to a fiber mount plate which is positioned relative to the laser and then solder to the base member. In the '253 patent, the transducer module has a flat substrate with a laser diode die soldered directly to the substrate. The optical fiber has a handling element adhered to it for positioning the fiber relative to the laser diode. The optical fiber is soldered directly to the substrate when optimum alignment is achieved between the fiber and the diode.
Another class of electro-optical transducer modules are designed for switching, modulating or phase-shifting an input optical signal. These modules have an electro-optic guided-wave transducer made of a crystalline material, such as lithium niobate, LiNbO.sub.3, lithium tantalate, LiTaO.sub.3, or the like, with an optical waveguide formed in one surface of the crystalline material and electrodes formed relative to the optical waveguide for providing an electrical signal to the transducer for performing the switching, modulating or phase-shifting function. Generally, the transducer is mounted on a substrate in the transducer module with the waveguide and electrode structure oriented opposite the substrate. FIG. 1 shows a Ti:LiNbO.sub.3 directional coupler developed by AT&T Bell Laboratories, Holmdel, N.J., and described in the IEEE Journal of Lightwave Technology, Volume LT-3, No. 1, February, 1985. The electro-optical directional coupler has a housing 10 with a support block 12 onto which is secured the LiNbO.sub.3 crystal 14. A titanium diffused waveguide directional coupler 16 is formed in one surface of the crystal 14 and an asymmetric coplanar stripline 18 is formed on the same surface for the electrode. The waveguide surface of the crystal transducer 14 is positioned face-up on the support block 12 within the housing 10. An in-plane side-feed arrangement is used to provide a RF and DC input 20 and termination 22 to the stripline electrode 18 via coaxial cables. Single mode fibers 24, 26 and 28 with 8 micron cores and 125 micron cladding diameters are pigtailed to three of the four input/output waveguides of the directional coupler 16. The fibers are permanently fixed in alignment to the waveguides using a silicon V-groove array technique. The fibers are first placed in the silicon V-groove array 30 and the end of the array is polished. The array is then aligned with the waveguide of the directional coupler 16 and bonded to the crystal 14 using UV cured epoxy having an index of refraction nearly matching the fiber. The fibers in the V-groove array 30 are laterally aligned to within 0.5 microns of the waveguides of the directional coupler 16. Strain relief for the fibers is provided by capturing the cables in the device housing.
The design of the above described directional coupler transducer module requires the precise formation of both the waveguide within the LiNbO.sub.3 crystal and the V-groove array containing the optical fibers. Maintaining tolerances in the submicron range would be very difficult in any large scale manufacturing effort for a design of this type.
What is needed is an electro-optical transducer module having an electro-optic guided-wave transducer wherein the submicron tolerances between the transducer and connecting optical waveguides are easily maintained on an individual waveguide-to-transducer basis while at the same time providing ease of manufacturing of the transducer module.